Balloon dilatation catheters are used for a variety of procedures in which a body lumen or vessel is dilated. For example, such catheters are used in angioplasty procedures in which a stenosed region of an artery, such as a coronary artery, is widened by inserting a deflated balloon into the stenosis and then inflating the balloon under pressure to enlarge forcibly the lumen through the artery. Such catheters typically have an elongate flexible shaft and a balloon mounted at the distal end of the shaft. The shaft has an inflation lumen that communicates from the proximal end of the catheter to the interior of the balloon at the distal end of the shaft. The catheter also has a main lumen that extends fully the length of the catheter shaft, terminating in a distal outlet at the distal tip of the shaft, beyond the balloon. The main lumen may be used to receive a guidewire as well as to provide fluid communication with the interior of the patient's artery to inject radiopaque dye into the artery to visualize it fluoroscopically or to monitor the pressure in the artery, distally of the stenosis. Typically, the balloon is inflated with a liquid which is radiopaque so that the configuration and action of the balloon may be monitored fluoroscopically during the angioplasty procedure. Use of an incompressible liquid as an inflation medium assures effective development and transmission of dilating forces to the balloon and to the stenosed region of the artery that is to be dilated.
Part of the preparation of the catheter for use involves purging air from the balloon and the inflation lumen to minimize the compressibility of the balloon fluid system. With some catheters, for example, as the type disclosed in U.S. Pat. No. 4,545,390 to Leary, a catheter first is evacuated by a syringe connected to the inflation lumen at the proximal end of the catheter. After air has been evacuated from the balloon, the inflation lumen and balloon are filled, by the syringe, with inflation liquid. Typically, one or more bubbles of air will remain entrapped in the balloon and, in an effort to purge as much air from the system as possible, it is the common practice to fill the balloon while holding the catheter with its distal end hanging down to permit the air to rise through the inflation lumen to the proximal end of the catheter where it may escape to atmosphere from the vented proximal end of the inflation lumen. Most, but not all of the air can be removed by this procedure. Usually, a small bubble of air will remain in the system.
A number of arrangements have been used to effect a more complete purge of air from the catheter. U.S. Pat. No. 4,323,071 discloses a dilatation catheter system in which a slender venting tube is passed through the inflation lumen and into the balloon. As inflation liquid is purged through the inflation lumen and into the balloon, air within the system is permitted to escape through the vent tube. After the system has been completely filled with inflation liquid and the air has been vented, the vent tube may be removed or sealed within the inflation lumen. This system for purging air from the catheter is somewhat time consuming and may be awkward in that it requires a number of manipulations of the vent tube. Additionally, there is some risk that the vent tube may damage the balloon. A further difficulty is that if a minute drop of liquid contacts the distal tip of the vent tube before purging is completed, the tube will become blocked by capillary action and may have to be replaced.
U.S. Pat. No. 4,684,363 to Ari discloses a balloon dilatation catheter having a pair of parallel inflation lumens both of which extend from the proximal end of the catheter through the catheter shaft into communication with the interior of the balloon. The catheter is filled with inflation liquid by directing the liquid through one of the inflation lumens into the balloon while permitting the other lumen to vent to the atmosphere to permit air to escape. After both inflation lumens and the balloon are filled with inflation liquid, both of the lumens are connected to the inflation/deflation device and are operated in parallel to inflate or deflate the balloon. This approach requires the use of an additional lumen which necessarily requires either that the catheter be increased in outer diameter or that the other lumens in the catheter be smaller in size, thereby diminishing their capacity. Additionally, such a three lumen catheter is more difficult to extrude particularly in the smaller sizes of such catheters.
Another approach for venting the balloon is disclosed in U.S. Pat. No. 4,692,200 in which a very small air vent passage is formed between the catheter shaft and the distal end of the balloon where the balloon is mounted to the shaft. The vent passage is small enough to permit slow leakage of air out of the balloon as inflation liquid is pumped into the balloon system but is too small to permit an undue amount of liquid to leak through the vent in the operating range of most pressures. Among the disadvantages of this purging arrangement is that it presents some risk to the patient in that before the catheter is inserted into the patient, it is deflated by aspirating through the inflation lumen. Typically, there is some delay from the time that the catheter is aspirated until the time it is actually inserted into the patient. During that time, air may be drawn into the balloon through the vent. When the balloon is thereafter inserted into the patient and inflated, the inflation pressure may drive the air out through the vent and into the patient's coronary artery, thus presenting a risk of an air embolism. Additionally, the very small vent results in a relatively slow purging procedure.
It is among the general objects of the invention to provide a catheter having an improved purge system that avoids the foregoing difficulties.